The present invention concerns the creation of concrete surfaces and more particularly the metal joint used to this effect in order to demarcate the slabs.
In making large concreted areas, the surface is divided into rectangular or square sections constituting the slabs of concrete. This division is generally achieved using metal profiles that demarcate each concrete slab and form the joints between slabs.
Advantageously, the joints are provided with means of absorbing the variations in dimensions of the slabs caused by temperature variations. These joints must also be able to absorb heavy loads while the correct level of the surface of the slabs and avoiding any degradation of the edges of the concrete slabs.
To this end, the joints must meet the following criteria:                offer efficient protection for the sharp edges of the concrete slabs;        guarantee positive anchoring in order to avoid any risk of separation from the slab;        allow the use of a sufficient thickness of material to avoid shearing of the slab due to weak points cased by the profile of the joint;        allow removal or expansion of the slabs by means of mortise and tenon interlocking, that also ensures that the level of the slabs is maintained.        
In general, these joints for concrete slabs are made from sheet steel profiles and more particularly of the double profile type with male and female interlocking such as mortise and tenon which allows expansion of the slabs and resists vertical displacements when heavy loads are applied.
A joint commonly used is made from a double profile of a material in the shape of an omega, in which the external shape of one matches the internal shape of the other. The male central part of the joint must necessarily present a sufficient volume to allow it to be filled with concrete when it is cast.
For a constant slab thickness and in the case where the upper part of the joint has to be augmented for reasons concerning its ability to transfer heavy loads, the lower part of the joint automatically becomes insufficient and as a result is no longer able to tolerate these loads because of lack of thickness of the matrix. As a consequence of this it is necessary to have numerous models of joint of different heights.
Another problem encountered with this type of profile is that, when the height of the concrete slab is limited, the minimum dimensions of the omega-shaped profile remain nevertheless very large because of the volume necessary for the central (male) part of the joint. As a result of this the mass of concrete that remains in the upper part of the edge of the slab, situated above the interlocking of the profile, is highly insufficient to be able to withstand the normal (vertical) loads on the surface of the slab and, consequently, this part is exposed to deterioration by cracking or spalling of the concrete.
At the present time joints are available with male and female interlocking offset below the median line of the slab in order to obtain a greater thickness of material above the interlock with a view to procuring greater strength for loads on the edges of the slabs.
Document WO 99/55968 also describes a structural joint for concrete slabs comprising, firstly, an L-shaped female profile of which the vertical wing extends along the length of the edge of the slab and as far as the upper edge of the latter and of which the double horizontal wing extends towards the interior of the slab and, secondly, an L-shaped male profile of which the vertical wing also extends along the length of the edge of the slab and as far as the upper edge of the latter and of which the horizontal wing extends towards the exterior of the slab so as to be able to engage with the female profile of the adjacent slab.
The problem encountered with this type of profile is that it is rolled in continuous lengths and, when it is placed in the concrete, it cuts the thickness of the slab into two parts in the vicinity of the joint. At this position there remains only half the thickness of the slab on either side of the male and female profile, which causes incipient failures in the longitudinal direction of the slab.
Although this joint offers good strength against vertical loads, one nevertheless observes incipient cracking at the extremities of the horizontal portions of the profiles due to the fact that these joints extend continuously over the whole length of the concrete slab while weakening the edges of the concrete. In effect, the thickness or height of the concrete slabs is calculated to withstand maximum vertical loads but the edges of the slabs no longer have all the height necessary to withstand these loads, given that they are interrupted over all of their length by the horizontal wing of the profile of the joint.
Another problem with this type of joint is that it offers only limited strength against deformations of the thin edge of the concrete slab, given that the thickness of the profile that extends as far as the upper surface remains limited to the thickness of the sheet metal forming the profile. It is important to use joints which procure efficient strengthening of the upper edge of the concrete slabs.
In general, structural joints for concrete slabs include firstly an L-shaped female profile of which the vertical wing extends along the length of the edge of the slab and as far as the upper edge of the latter and, secondly, an L-shaped male profile of which the vertical wing also extends along the length of the edge of the slab and as far as the upper edge of the latter, extending continuously over the whole length of the slab.
These two profiles are assembled facing each other so as to form the reinforced lips of the concrete slabs to be joined. These metal joints are heavy and expensive.